This invention relates to a centrifugal pump utilizing a multiple disk rotor for pumping a fluid. In one of the more specific aspects of the invention, a plurality of vanes are combined with the multiple disks of the rotor.
Centrifugal pumps have been known for a number of years. In fact centrifugal pumps utilizing a vaned rotor have had wide commercial success because of the durability, low cost and high efficiency of such pumps. In centrifugal pumps a fluid is forced to circulate around a given point, this circulation of a fluid is called vortex circulation. During the circulation of the fluid a radial pressure gradient is created in the fluid. The gradient is such that the pressure increases with increasing radial distance from the center of rotation. The rate of the pressure increase depends upon the speed of rotation of the vaned rotor and the density of the fluid being pumped. An external force must act on the fluid to create the vortex circulation. The force must accelerate the fluid in the tangential direction, as the fluid moves outward, in order to maintain the angular velocity of the fluid. The force supplied to the fluid transfers momentum to the fluid. In a pump using a conventional vaned rotor the vanes and rotor walls form a channel for the fluid. As the channel is rotated the fluid accelerates as it moves outwardly into regions of higher rotor velocity. The acceleration of the fluid in the channel transfers momentum to the fluid. The conventional pump utilizing a vaned rotor has been especially successful in moving low viscosity fluids at a high flow rate.
However, there are a number of deficiencies associated with the pump using a vaned rotor. These deficiencies seriously limit the application range for such pumps.
Most of the difficulties associated with a pump utilizing a vaned rotor occur at the inlet region where the fluid is first introduced into the pump. The impact of these difficulties are that a vaned rotor pump can have cavitation problems, a low efficiency when pumping viscous fluids and a low resistance to wear when pumping abrasive fluids. Although some of these deficiencies can be overcome by modifications to the pumping system such modifications are usually expensive and limit the performance of the pump.
When the vanes on a rotor and a pump move through a fluid they produce a pressure distribution that has a positive pressure on the advancing face of the vane and a negative pressure on the retreating face. The intensity of the negative pressure zone depends on the radial flow velocity of the fluid along the vanes and the velocity at which the rotor is rotating. This type of pressure distribution is inherent in a pump utilizing a vaned rotor. Cavitation can occur in the negative pressure zone in the area having the lowest static pressure. In a vaned rotor, the lowest pressure is at the fluid inlet, and more specifically on the retreating side of the vanes at the fluid inlet. If the static pressure on the fluid in the pump drops below the vapor pressure for the fluid, vapor pockets will be formed. Cavitation occurs when such vapor pockets are formed in the rotor of the pump. Of course, cavitation severely restricts the performance of the pump. Also, since cavitation occurs at the fluid inlet to the pump, cavitation difficulties will impair the operational efficiency of the entire vaned rotor pump.
The only way to prevent cavitation is to provide enough inlet pressure so that even the low pressure areas at the fluid inlet to the rotor have sufficient pressure so that the static pressure is higher than the vapor pressure of the fluid. However, it is very expensive to provide sufficient inlet pressure to the pump to suppress cavitation. Also the environment in which the pump is being used may not allow for modifications to increase the inlet pressure to a point that is sufficient to suppress cavitation.
Viscous fluids also adversely effect the performance of a pump using a vaned rotor. The difficulty occurs because there is a non-uniform pressure distribution on the vanes of the rotor. The non-uniform pressure distribution occurs at the inlet region of the pump where the viscous fluid is first engaged by the vanes of the rotor. The fluid flow interacting with the vanes of the rotor generate Karman Vortices along the retreating face of the vanes. The vortices represent lost momentum that could have been used to pump the fluid. The loss of momentum occurs in this type of pump regardless of the viscosity of the fluid, but the effects of this loss of momentum are more severe with viscous fluids. Thus, a pump utilizing a vaned rotor has reduced efficiency when pumping viscous fluids.
When pumping abrasive fluids the rate of abrasion is a function of a type of concentration of the particles in the fluid and the relative velocity between the surface of the rotor and adjacent fluid layer. There is a layer of relatively quiescent fluid, called the boundary layer, adjacent to the surfaces of the rotor. The thickness of the boundary layer is mainly determined by the Reynolds number of the fluid. The boundary layer will provide a protective layer of fluid that helps to prevent the particles in the abrasive fluid from coming in contact with the surface of the rotor. However, the effectiveness of the boundary layer is significantly reduced when the thickness of the boundary layer is decreased.
In a pump utilizing a vaned rotor the fluid being pumped undergoes an abrupt acceleration and change of direction as the fluid enters the rotor. The changes in acceleration and direction of flow of the fluid act to reduce the thickness of boundary layer. As the boundary layer is reduced in thickness the particles of the fluid pass across the rotor surface at approximately the velocity at which the fluid is traveling. This produces a strong abrading action on the surface of the rotor. Again the effects of the abrasive fluids are greatest at the inlet region of the rotor where the fluid undergoes abrupt acceleration and changes of direction. Thus, when pumping abrasive fluids the inlet region of the rotor will receive the most damage and be the first area of the rotor to fail.
From the above it is clear that a pump utilizing a traditional vaned rotor is significantly limited in application by the inlet conditions inherent in such a pump. These limitations significantly reduce the areas of application for such pumps.
Another type of centrifugal pump that has been known is the multiple disk pump. This pump was originated by Nikola Tesla and he was granted a patent (U.S. Pat. No. 1,061,142) in 1912 on this pump concept. This pump utilizes a plurality of rotating disks as the rotor for the pump. The rotating disks utilize viscous drag to transfer momentum to the fluid to be pumped. Viscous drag results from the natural tendency of a fluid to resist flow. Viscous drag occurs whenever a velocity difference exists between a fluid and the constraining channel in which the fluid is located. Viscous drag always acts to reduce the velocity difference between the fluid and the moving channel or the rotor.
Although the Tesla multiple disk pump has been known for a number of years the pump has never been commercialized or seriously pursued in the pump industry. At least part of the reason for this lack of development of the Tesla pump is that there are some significant performance limitations with this type of pump. The efficiency of the multiple disk rotor decreases at higher flow rates for the pumped fluid. In addition, a relatively large number of disks are required to achieve pump efficiency when a low viscosity fluid is being pumped. The number of disks required has a direct relationship to the manufacturing costs of the rotor and casing for the pump. Also the multiple disk rotor is not inherently rugged. The disks are usually constructed from a relatively thin material but this material must be stiff enough to prevent flexure during the operation of the pump. In view of these limitations the Tesla type multiple disk rotor pump has never been effectively commercialized.